Sensors based on different types of guiding modes have been used for sensor applications for a long time. Those technologies were commercialized and actively used in different sensing applications. However, new challenges in biomedical research are requiring even better sensors. One of those new perspective technologies is plasmonic hyperbolic metamaterials. It was shown that those structures have a big potential for sensing. In order to compare these new guiding structures with traditional ones, we performed the analysis of sensitivity for different guiding structures: optical dielectric waveguide, surface-plasmon polaritons, long-range surface plasmon polaritons (LRSPP), plasmonic hyperbolic materials with a combination of metal and dielectric layers. All structures were placed on the BK7 glass substrate and we set wavelength at 1550 nm. We used the Si3N4 layer a waveguiding medium for the dielectric waveguide, we used gold for all plasmonic structures. The water layer on the top of all structures was used as a sensing area. For guiding modes coupling we used diffraction gratings for a few reasons. Firstly, there are no materials with a refractive index capable to couple guiding modes. Secondly, diffraction gratings provide compact, a planar design which is easier to keep the structure in the thermostatic condition And last but not least, we used coupling gratings with the same grating profile (sinusoidal) and the same corrugation depth so all guiding modes will be coupled the same way. Our results showed that plasmonic hyperbolic structures indeed have much higher sensitivity comparing with the traditional guided wave sensors based on dielectric waveguides and surface plasmon-polaritons.
There are many applications which require high sensitivity spectral detection. In some cases, you need the wavelength range to be extended to cover all necessary spectral fingerprints. We are proposing a broadband spectrometer for ultrasensitive detection based on plasmonic hyperbolic metamaterials and diffraction gratings. Using variety of materials in fabrication of the hyperbolic metamaterials, we can cover the wide spectral range from near UV (~250 nm) to IR (~2 μm). In our spectrometer, the diffraction gratings have two functions. One is coupling the incident light source with the plasmonic guiding modes, which have a very high effective refractive index (≥8.1), much higher than the refractive index of germanium (4.05), the natural material with the highest refractive index. While a prism can also be used for coupling guiding modes with incident light, a diffraction grating is the only way to excite the guiding modes because of the plasmonics modes with very high effective refractive index. The second function of the diffraction gratings is their natural role in spectrometers. We demonstrated based on numerical simulations that we could reach high detection spectral sensitivity using compact diffraction gratings combined with hyperbolic metamaterials; the huge “n-meter” spectrometer is not necessary.
Plasmonic structures for biomedical sensing are in use for a long time. However, there is a fundamental limitation of their sensitivity due to low effective refractive index of layered plasmonic structures. We are proposing a hyperbolic metamaterial (HMM) structure which is a combination of surface plasmon Polaritons (SPPs) and long-range surface plasmon Polaritons (LRSPPs) modes. The result of the interaction between these modes leads to plasmonic modes with ultra-high effective refractive index. We calculated and optimized plasmonic HMM structure with effective refractive index equal to 8.1, i.e. twice as much as that of germanium, a natural material with the highest refractive index. We simulated these structures for gold, silver, copper and aluminum. The best way to use these structures for protein sensing is to use diffraction gratings – there is no natural material which can be used as a prism. By optimizing layer parameters and diffraction grating we were able to build a model of the structure with sensitivity as 10-9 for refractive index. We are hoping to achieve sensitivity up to 10-11, so this structure can be used for different protein sensing application including detection of metastatic cells spreading the human body.
We report U-shaped biconically tapered optical fibers (BTOF) as dip probes for label-free immunoassays. The tapered
regions of the sensors were functionalized by immobilization of immunoglobulin-G (Ig-G) and tested for detection of
anti-IgG at concentrations of 0.5, 5.0, and 50 μg/mL. Antibody-antigen reaction creates a biological nanolayer
modifying the waveguide structure leading to a change in the sensor signal, which allows real-time monitoring. The
kinetics of the antibody (mouse Ig-G) -antigen (rabbit anti-mouse IgG) reactions was studied. The limit of detection for
the sensor was estimated to be less than 0.5 μg/mL with low temperature sensitivity. Utilization of the rate of the sensor
peak shift within the first few minutes of antibody-antigen reaction is proposed as a rapid detection method.
A single biconical fiber taper is a simple and low-cost yet powerful sensor. With a distinct strength in refractive index (RI) sensing, biconical tapered fiber sensors can find their place in handheld sensor platforms, especially as biosensors that are greatly needed in health care, environmental protection, food safety, and biodefense. We report doubling of sensitivity for these sensors with two passes through the tapered region, which becomes possible through the use of sensitive and high-dynamic-range photodetectors. In a proof-of-principle experiment, we measured transmission through the taper when it was immersed in isopropyl alcohol-water mixtures of varying concentrations, in which a thin gold layer at the tip of the fiber acted as a mirror enabling two passes through the tapered region. This improved the sensitivity from 0.43 dB/vol % in the single-pass case to 0.78 dB/vol % with two passes through the taper. The refractive index detection limit was estimated to be ~1.2×10−5 RI units (RIU) and ~0.6×10−5 RIU in the single- and double-pass schemes, respectively. We predict that further enhancement of sensitivity may be achieved with a higher number of passes through the taper.
In undergraduate optics laboratory, one thing that is not easily achieved is quantitative measurement of optical phase. The reason is that optical phase measurement usually requires expensive interferometers. We demonstrate measurement of relative optical phase shift upon total internal reflection. Total internal reflection, though known by every student of optics, is remembered by 100% reflection at an interface when angle of incidence is greater than the critical angle, that is, it seems all the same beyond the critical angle. This is not entirely true if one considers the optical phase, which keeps changing upon total internal reflection as the angle of incidence is varied. Furthermore, for linear polarization states perpendicular to or in the plane of incidence (s- and p- polarization), optical phase changes differently upon total internal reflection. Therefore, a linearly polarized beam composed of both s- and p- polarization undergoing total internal reflection becomes elliptically polarized. We show how to determine relative optical phase change between s- and p- polarization states through analysis of the outgoing elliptically polarized beam. Such optical phase change can also be theoretically calculated using Fresnel equations.
Generation of picosecond pulses at two distinct wavelengths is interesting for wavelength-division-multiplexing, fiber communication and sensing. For this purpose, we achieved harmonic active mode locking simultaneously at two wavelengths separated by about 15 m in an Erbium-doped fiber laser. Dual- wavelength lasing was obtained with two wide-bandwidth (greater than 1 nm) nonchirped high-reflectivity fiber Bragg gratings inserted in the laser cavity. The fiber Bragg gratings were written with 275-nm light from an Ar laser in hydrogen-loaded fibers. Optical path lengths and losses were carefully adjusted at each wavelength to obtain perfect mode locking at both wavelengths. Total cavity dispersion was set in the anomalous dispersion regime and optimized at each wavelength independently to generate solitons. Pulses at 3-GHz repetition rate were obtained at two wavelengths simultaneously with pulse widths of 16 ps and 13 ps, at 1547 nm and 1562 nm respectively. Time-bandwidth products of 0.37 and 0.34 respectively confirmed that the pulses were nearly transform-limited at each wavelength.